It has long been known to use a system of lighting instruments, where each are equipped with a different color filter to illuminate an area with a light having an overall desired hue and saturation. Many mechanical devices have been utilized with a limited degree of success to move color filters into and out of a light beam to vary the color of a single instrument, thereby reducing the number of instruments required. The color changer disclosed in U.S. Pat. No. 4,600,976 illustrates such a mechanism. Another example of a light source having automatically variable hue, saturation and beam divergence is disclosed in U.S. Pat. No. 4,602,321. Other examples of commercially available color changers include: a Gel Jet scrolling gel changer made by Fine Arts Technologies of Eureka, Calif.; and a Crystal Color electronic color changer made by Artifex Corporation of Costa Mesa, Calif. Lighting instruments utilizing color filters such as those noted above operate on the principle of subtracting certain wavelength light rays from a white light beam to thereby produce a resultant beam having the desired color. Generally, color producing mechanisms which have color filters arranged in series are of the subtractive type.
An alternate, well-known approach for producing a light beam of the desired color is to arrange in parallel plural colored light sources, and combine the multiple primary colored light beams in the proper proportions to obtain the overall desired color. A variable color floodlight employing such an additive coloring technique is disclosed in U.S. Pat. No. 4,535,394. The floodlight of the noted patent is configured with dichroic mirrors arranged either as a pair of crossed dichroic filters, or as a four-sided pyramid with criss-crossed dichroic mirrors in the center thereof. This arrangement is utilized to combine three primary color light beams into one composite light beam yielding the desired overall color. In U.S. Pat. Nos. 3,825,335 and 3,825,336 there is disclosed photographic apparatus employing optical fibers arranged in bundles for combining three primary colored light beams into one or two resultant composite light beams. Yet another additive color changer disclosed in U.S. Pat. No. 4,314,318 employs a multi-faceted light-reflecting member designed to combine three primary colored light beams, radially oriented 120 degrees apart, into a resultant composite light beam. These systems all employ three primary colored light beams which are produced by respective white light sources, in conjunction with suitable fixed color filters and an additive mixer.
The generation of colored light beams is generally carried out by varying the intensity of one or more primary colors and mixing the result together. While this technique is effective to achieve many variations in color, certain shortcomings arise. When two primary colors are mixed together in accordance with the noted technique, for example a red color and a green color, the common spectral wavelengths of the two primary colors predominate and yield a yellow color. However, because the centerline wavelengths of the red and green colors remain unchanged, the resultant yellow color contains vestige red and green spectral wavelengths and thus the saturation of the yellow color is diminished.
Another problem particularly prominent in additive color mixing systems is the reduced efficiency when it is desired to produce an output white light by adding the filtered light of the primary colors. The reason is that the power available from the three primary color sources is limited by the color filters used with each such light source. When the three primary colors are combined, each source is only contributing a fraction of its total energy to the resultant composite beam, the remainder being absorbed or reflected by the filter itself. This can be appreciated as the total output of the three sources, combined in the appropriate proportions, produces less power than a single, unfiltered source.
It is also difficult to obtain a high quality white light beam because color balance is very critical. Highly saturated colors are very difficult to obtain since the filtered primary color represents the most saturated hue available from the light source. The primary colors are usually utilized because they are easily used to produce many other colors.
The stage lighting instrument described in U.S. Pat. No. 4,602,321, assigned to the assignee hereof, provides independent adjustment of the hue and saturation of a light beam. The hue adjustment includes a spectrum of visible light ranging from the violet to the red, and also includes combinations of red and violet yielding shades of magenta, lavender, and pink. The continuous range of saturation adjustment includes deeply saturated hues, pastel shades, and pure white light. However, the subtractive system of the noted patent includes three sets of pivoting dichroic color filters arranged in series such that a beam of light sequentially passes through the first filter set, then passes through the second filter set, and finally passes through the third filter set. The filter properties and angle of orientation, while chosen to yield a useful range of colors, also yield certain redundant filter angles of incidence with respect to the light beam. What this means is that if the first filter set were a set of short-wave-pass blue filters rotated to a position which passes light with wavelengths in the range of 400 to 500 nanometers (nm), rotation of a second set of long-wave-pass red filters acting on light having wavelengths in the range of 600 to 700 nm, would have no effect on the resultant color of the light beam. This is because the first filter set effectively rejects or eliminates wavelengths in the 600 to 700 nm range before the light beam reached the second filter set.
In an additive color mixing system, each light source is modified separately (usually only the intensities of the light sources are modified) before the light emanating therefrom is combined. Subtractive systems, in which color filters are arranged in series, are designed and operated with the particular limitation in mind that the first filter in the series has primary control over the white light, thereby constraining the subsequent filters to operate only upon the remaining wavelengths which are allowed through the first filter. This limitation, while quite significant when considering the dichroic color balance system disclosed in U.S. Pat. No. 3,085,468, is not as great in the lighting instrument disclosed in U.S. Pat. No. 4,602,321, since any of the filter sets in the stage lighting instrument can be rotated to positions parallel to (and thus not affecting) the light beam. Still, there are certain filter combinations in which the precise position of a certain filter set may be irrelevant since the filter set has no effect on the light beam. In an additive color mixing system, each of three primary color filters acts upon a different white light beam, whereby the position of a pivoting dichroic color filter placed in the respective beam can be precisely controlled to obtain the desired resultant color.
It can be seen from the foregoing that a need exists for a high-performance additive color mixing system which overcomes the disadvantages of the devices known in the prior art. A particular need exists for a color mixing system which overcomes the limited range of hue and saturation control resulting from the use of fixed color filters. A need exists for a color mixing system which provides highly saturated colors by moving the centerline wavelengths of two primary colors together to reduce the vestige wavelenghts. Another need exists for a color mixing system which overcomes the problem of limited intensity in white light resulting from the energy wasted by the primary color filters. Yet another need exists for a color system which solves the inherent problem of the inability to produce a true white light beam.